When using a semiconductor single crystal wafer, e.g., a silicon wafer to fabricate a device, many processes from a wafer machining process to a device forming process intervene, and one of such processes is a heat treatment process. The heat treatment process is an important process which is carried out for the purpose of, e.g., formation of a defect-free layer on a surface layer of a wafer, gettering, crystallization, formation of an oxide film, or diffusion of an impurity.
As a diffusion furnace (an oxidation/diffusion apparatus) that is used for such a heat treatment process, e.g., oxidation or impurity diffusion, a vertical heat treatment furnace in which a heat treatment is carried out in a state where many wafers are horizontally held at predetermined intervals is mainly used with an increase in diameter of wafers. Further, when performing a heat treatment to the wafers by using the vertical heat treatment furnace, a vertical heat treatment boat (which will be referred to as a “heat treatment boat” or simply referred to as a “boat” hereinafter) that is required to set many wafers is utilized.
FIG. 8 shows an outline of a conventional general vertical heat treatment boat 210. A pair of plate-like members (which will be also referred to as coupling members or a top plate and a bottom plate) 216 are coupled with both end portions of each of four supports (rods) 214. Many grooves 211 are formed on each support 214, and a convex portion between the respective grooves 211 functions as a support portion 212 for a wafer. When performing a heat treatment to each wafer, as shown in a plan view of FIG. 9(A) and a front view of FIG. 9(B), each wafer W is horizontally supported by mounting an outer peripheral portion of the wafer W on the support portions 212 formed at the same height of each support 214.
FIG. 10 is a schematic view showing an example of a vertical heat treatment furnace. Many wafers W are horizontally supported on a heat treatment boat 210 installed in a reaction chamber 222 of a vertical heat treatment furnace 220. At the time of a heat treatment, each wafer W is heated by a heater 224 provided around the reaction chamber 222. During the heat treatment, a gas is introduced into the reaction chamber 222 through a gas introduction tube 226 to flow downwardly from an upper side and discharged toward the outside from a gas exhaust tube 228. Although the gas to be used differs depending on a purpose of the heat treatment, H2, N2, O2, Ar, or the like is mainly used. In case of impurity diffusion, each of these gases is used as a carrier gas for an impurity compound gas.
Various kinds of shapes are adopted for the wafer support portion 212 in the vertical heat treatment boat 210, and each of FIGS. 11(A) and (B) shows an example. In (A), concave grooves 211 are provided on the support 214 having a semicylindrical shape to form semicircular support portions 212. On the other hand, in (B), concave grooves 211 are provided on a wide prismatic support 215 to form rectangular support portions 213 in order to support each wafer W at a position closer to the center thereof than (A). Besides, a groove shape may be an arched shape or a hook-like shape.
Furthermore, a structure where relatively large plate-like support portions (support plates) are provided on each support to support each wafer in a stable state (see Japanese Unexamined Patent Publication (Kokai) No. 2000-53497) or a structure where a step is provided on an upper surface of each support portion to enable supporting wafers having different diameters (see Japanese Unexamined Patent Publication (Kokai) No. 2005-159028) is also suggested.
In regard to a material of the boat, a material such as quartz (SiO2), a silicon carbide (SiC), or silicon (Si) is usually utilized for, e.g., a silicon wafer in order to avoid contamination of wafers. For example, in a heat treatment process at a high temperature exceeding 1000° C., an SiC or Si boat having higher heat resistance than a quartz (SiO2) boat is used. In particular, the SiC boat is often used since metal contamination that occurs during a heat treatment can be more reduced when the SiC boat is subjected to CVD-SiC coating.
Meanwhile, in case of using the vertical heat treatment boat, internal stress due to a weight of the wafer itself, thermal strain stress due to non-uniformity of an in-wafer temperature distribution or the like occurs when a high-temperature heat treatment is performed for the purpose of oxidation, impurity diffusion or the like in particular, and slip (slip dislocation) as a crystal defect occurs in the wafer when such stress exceeds a given fixed critical value. Since the critical value for occurrence of the dislocation is precipitously reduced when a temperature rises, it is known that the slip dislocation is apt to occur as a temperature increases. When a device is formed at a position where the slip dislocation occurs, which may be a factor of, e.g., junction leakage, thereby a yield ratio of fabrication of a device considerably decreases in some cases.
It is known that occurrence of this slip dislocation becomes harder to be suppressed when a diameter of the wafer is increased. When a wafer load rises due to an increase in diameter and, for example, three-point support that three support portions support the wafer is adopted, the wafer load is apt to be concentrated on one of the three support portions, and the slip dislocation readily occurs. Thus, since the wafer load must be distributed to suppress occurrence of the slip dislocation, a vertical heat treatment boat in which four or more support portions are provided as shown in FIGS. 8 and 9 in place of the three-point support to enable support at four or more points is utilized.